Motor controller

ABSTRACT

An electric motor control apparatus of this invention estimates changes in temperature of a semiconductor device to compute temperature change amplitude  108  based on an output current signal  105  computed from a current flowing through the semiconductor device of a switching circuit  5 , an operating frequency signal and a carrier frequency signal by a temperature change estimation part  11 , and makes conversion into the number of power cycles  110  corresponding to the temperature change amplitude  108  from power cycle curve data stored in a power cycle curve data storage part  14  and computes a thermal stress signal  111  by a thermal stress computation part  13 , and does life estimation of the semiconductor device based on the thermal stress signal  111  and produces an output to a display part  16  as a life estimation result signal  112  by a life estimation part  15   a.

TECHNICAL FIELD

This invention relates to an electric motor control apparatus forperforming variable speed control of an electric motor.

BACKGROUND ART

In an electric motor control apparatus such as an inverter apparatususing a semiconductor device for electric power, at the time of anoperation of the apparatus, the semiconductor device for electric powergenerates heat and a junction temperature of a chip increases and at thetime of a stop of the apparatus, the heat generation stops and thejunction temperature decreases. Also, at the time of starting andstopping an operation of an electric motor and the time of a suddenchange in a load, an output current of the apparatus changes largely, sothat the junction temperature of the semiconductor device for electricpower also changes largely. As a result of this, thermal expansion andthermal shrinkage of a chip portion of the semiconductor device forelectric power are repeated due to repeats of the operations and stopsand repeats of the sudden changes in the load or speed.

On the other hand, the semiconductor device for electric power isgenerally assembled using various materials with different thermalexpansion coefficients, so that particularly when a temperature of ajunction between a heat spreader and a wire bonding portion or thesemiconductor device for electric power increases, wire bondinggradually begins to peel off a chip due to thermal expansion stress of acoating agent of the chip or metal fatigue of a junction material beginsto occur due to thermal expansion stress by a difference between thechip and the heat spreader in thermal expansion coefficients. Due torepeats of the operations and stops and repeats of the sudden changes inthe load or speed, finally, the wire bonding peels completely andbecomes an open state. That is, it results in failure or breakage of thesemiconductor device for electric power. A cycle of thermal expansionand thermal shrinkage to the time when this wire bonding peelscompletely due to thermal expansion stress and results in failure orbreakage is called a power cycle.

Therefore, particularly in an apparatus with high repeating frequency ofthe operations and stops, for example, in the case of using the inverterapparatus in motor driving of an elevator or an AC servo apparatus, orin an apparatus with sudden changes in the load, for example, in thecase of using the inverter apparatus in a compressor, a life of thesemiconductor device for electric power shortens due to the power cycle,so that some measures need to be taken.

There is a life monitoring apparatus of a semiconductor device forelectric power disclosed in Patent Reference 1 (JP-A-8-51768) as meansin which an object is to provide an apparatus capable of monitoring alife of a semiconductor device for electric power due to a power cycleand grasping time of maintenance of the semiconductor device forelectric power before the life expires and preventing breakage of thesemiconductor device for electric power.

The object of Patent Reference 1 is to protect the semiconductor devicefor electric power used in an inverter apparatus etc. before resultingin the life due to the power cycle, and it is constructed so that from acorrelation between a power cycle and a difference between junctiontemperatures of the semiconductor device for electric power, a powercycle corresponding to the difference between junction temperatures ofthe semiconductor device for electric power in the inverter apparatus isestimated to be the life and the number of operations of the inverterapparatus is counted by a counter and when a count value exceeds a firstreference value, an alarm signal is outputted and when the count valueexceeds a second reference value, a trip signal is outputted and theinverter apparatus is stopped forcedly.

In Patent Reference 1, there was a problem that a value of thedifference between junction temperatures changes normally but the valueof the difference between junction temperatures is set to a fixed valueby selecting typical one point in operations and stops of the apparatusand desired accuracy of an estimated life cannot be obtained.

Also, there is Patent Reference 2 (JP-A-8-126337) as means in which anobject is to obtain an inverter apparatus capable of takinglife-prolonging measures, for example, improving a use method beforereaching a life of a semiconductor device for electric power.

Performing alarm processing such as an alarm display command when thenumber of thermal stresses obtained from the number of thermal stressesof temperature change amplitude computed based on amplitude in changesin an estimated temperature of the semiconductor device for electricpower and the number of thermal stresses of temperature change ratiocomputed based on ratio in changes in an estimated temperature of thesemiconductor device for electric power exceeds the number of allowablethermal stresses and also, obtaining the residual life time from thenumber of thermal stresses and the number of allowable thermal stressesto execute a display command and also, performing alarm processing suchas an alarm display command since operation cannot be performed byexpected life time with operation of set time when the number of thermalstresses every set time obtained from the number of thermal stresses oftemperature change amplitude every set time and the number of thermalstresses of temperature change ratio every set time exceeds the numberof allowable thermal stresses per set time and also, obtaining anoperable life with operation of set time to execute a display commandare described in Patent Reference 2.

In Patent Reference 2, it is constructed so that a part which hasreached fatigue is displayed by life estimation due to thermal stressand a worker can easily decide the part to prevent a fault and also byestimating whether or not operation can be performed by expected lifetime with operation of set time, the worker can improve a use method ora load state and use frequency of the inverter apparatus and therebytake life-prolonging measures, but unless the worker inspects a displaypart and checks its display or alarm and checks a life determinationresult or a life estimation result, the life-prolonging measures cannotbe taken and there was also a problem that an abnormal stop of a systemis made since the inverter apparatus stops an output by a lifedetermination without taking the life-prolonging measures in case ofmissing the display or alarm.

This invention is implemented to solve the problems as described above,and a first object is to obtain an electric motor control apparatuscapable of doing life estimation with high accuracy.

Also, a second object is to obtain an electric motor control apparatuscapable of satisfying a set expected life by automatically decreasingamplitude of changes in temperature of a semiconductor device.

DISCLOSURE OF THE INVENTION

In an electric motor control apparatus for converting DC electric powerinto AC electric power of a variable frequency and a variable voltageand performing variable control of an electric motor acting as a load,having a switching circuit having a semiconductor device such as a powertransistor and a diode connected in parallel with this power transistor,a control part for generating a driving pulse based on an operatingfrequency signal set by an operating frequency setting part and acarrier frequency signal set by a carrier frequency setting part, and adriving circuit for amplifying the driving pulse outputted from thiscontrol part and performing on-off control of the power transistor ofthe switching circuit, the electric motor control apparatus of thisinvention comprises a current computation part for computing an outputcurrent from a current flowing through the semiconductor device and alsooutputting a current breaking signal to the control part when an outputcurrent signal computed exceeds a current limit value signal outputtedfrom a current limit level adjusting part, a temperature changeestimation part for estimating changes in temperature of thesemiconductor device to compute temperature change amplitude based onthis output current signal, the operating frequency signal and thecarrier frequency signal, a power cycle curve data storage part forstoring power cycle curve data showing a relation between thetemperature change amplitude and a power cycle life of the semiconductordevice, a thermal stress computation part for converting the temperaturechange amplitude computed by the temperature change estimation part intothe number of power cycles used as the power cycle life of thesemiconductor device by the power cycle curve data and computing athermal stress signal, and a life estimation part for doing lifeestimation of the semiconductor device based on this thermal stresssignal and producing an output to a display part as a life estimationresult signal and further calculating life time per set time andcomparing the life time with an expected life and outputting an alarm tothe display part as a life determination signal when the life time isshorter than the expected life, so that life estimation with highaccuracy can be done.

Also, since it is constructed so that the life estimation part outputsthe life estimation result signal and the life determination signal tothe current limit level adjusting part and also the current limit leveladjusting part makes an automatic adjustment so as to decrease a currentlimit value signal outputted to the current computation part when alarminformation is included in the life estimation result signal or the lifedetermination signal is inputted, amplitude of changes in temperature ofthe semiconductor device can be decreased and an expected life set canbe satisfied.

Further, since it is constructed so that the life estimation partoutputs the life estimation result signal and the life determinationsignal to the carrier frequency setting part and also the carrierfrequency setting part makes an automatic adjustment so as to lower anupper limit value of the carrier frequency and outputs a carrierfrequency signal to the control part when alarm information is includedin the life estimation result signal or the life determination signal isinputted, amplitude of changes in temperature of the semiconductordevice can be decreased and an expected life set can be satisfied.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a configuration of an electric motor controlapparatus according to a first embodiment of this invention.

FIG. 2 is a graph showing one example of steady-state losscharacteristics of semiconductor devices such as a power transistor anda diode.

FIG. 3 is a graph showing one example of switching loss characteristicsof semiconductor devices such as a power transistor and a diode.

FIG. 4 is a graph showing one example of changes in temperature of asemiconductor device.

FIG. 5 is a graph showing one example of characteristics of a powercycle curve stored in a power cycle curve data storage part 14 in theelectric motor control apparatus according to the first embodiment.

FIG. 6 is a diagram showing a configuration of an electric motor controlapparatus according to a second embodiment of this invention.

BEST MODE FOR CARRYING OUT THE INVENTION

First Embodiment

FIG. 1 is a diagram showing a configuration of an electric motor controlapparatus according to a first embodiment of this invention. In thediagram, a control part 1 generates a driving pulse 102 based on anoperating frequency signal 100 set by an operating frequency settingpart 2 and a carrier frequency signal 101 set by a carrier frequencysetting part 3 a, and outputs the driving pulse to a driving circuit 4.The driving circuit 4 generates a driving pulse 103 amplified by thedriving pulse 102 and performs on-off control of power transistors 6from which a switching circuit 5 is constructed and thereby, DC electricpower is converted into AC electric power of a variable frequency and avariable voltage and variable control of an electric motor 8 acting as aload is performed. Also, numerals 7 are diodes connected in parallelwith the power transistors 6.

Also, a current computation part 9 computes an output current from acurrent detection signal 104 detected by a current detector 10, andoutputs an output current signal 105 to a temperature change estimationpart 11. Also, the current computation part 9 compares the computedoutput current signal 105 with a current limit value signal 106 set by acurrent limit level adjusting part 12 a in order to protect the electricmotor 8 and semiconductor devices such as the power transistors 6 andthe diodes 7 from an overcurrent and when the output current signal 105exceeds the current limit value signal 106, a current breaking signal107 is outputted to the control part 1.

Also, based on the operating frequency signal 100 set by the operatingfrequency setting part 2, the carrier frequency signal 101 set by thecarrier frequency setting part 3 a and the output current signal 105computed by the current computation part 9, the temperature changeestimation part 11 estimates changes in temperatures of thesemiconductor devices such as the power transistors 6 and the diodes 7and computes temperature change amplitude.

Temperature change estimation processing in the temperature changeestimation part 11 will be described.

One example of steady-state loss characteristics of the semiconductordevices such as the power transistors and the diodes is shown in FIG. 2.In FIG. 2, the axis of abscissa is an output current I (the outputcurrent signal 105 computed by the current computation part 9) and theaxis of ordinate is a steady-state loss Ps. The steady-state loss Ps hascharacteristics increasing with increasing the output current I.

Also, one example of switching loss characteristics of the semiconductordevices such as the power transistors and the diodes is shown in FIG. 3.In FIG. 3, the axis of abscissa is an output current I (the outputcurrent signal 105 computed by the current computation part 9) and theaxis of ordinate is a switching loss Psw. The switching loss Psw hascharacteristics increasing with increasing the output current I.

First, using the output current signal 105 (described as the outputcurrent I in FIG. 2 and FIG. 3) computed by the current computation part9, the steady-state loss Ps is obtained by the steady-state losscharacteristics shown in FIG. 2 and the switching loss Psw is obtainedby the switching loss characteristics shown in FIG. 3. Next, from thesteady-state loss Ps and the switching loss Psw, a carrier frequency fcwhich is the carrier frequency signal 101 set by the carrier frequencysetting part 3 a, and a computation period Δτ, a heating value Q of thesemiconductor devices such as the power transistors 6 and the diodes 7from which the switching circuit 5 is constructed is obtained by Formula(1).Q=Ps(I)+Psw(I)×fc×Δτ  (1)

A temperature change amount Δθ for computation time Δτ can be obtainedfrom the heating value Q of the power transistors 6 and the diodes 7 andtransient thermal resistance Rth(t) defined by a mounted state of thepower transistors 6 and the diodes 7, and the temperature change amountΔθ is summed and computed to obtain a temperature change and a minimumpoint and a maximum point of this change are extracted and temperaturechange amplitude ΔT1 is calculated and is outputted to a thermal stresscomputation part 13 as a temperature change amplitude signal 108.

One example of changes in temperature of the semiconductor devices isshown in FIG. 4. In FIG. 4, Δt is set time, and ΔT1, ΔT2, ΔT3, ΔT4 andΔTn are temperature change amplitudes, and ΔTmax is the maximumtemperature change amplitude at the set time Δt. A cycle of thermalexpansion and thermal shrinkage to the time of resulting in failure dueto a cycle of an increase in temperature of this semiconductor device iscalled a power cycle.

The thermal stress computation part 13 computes a power cycle lifecorresponding to a temperature change amplitude signal 109 as a powercycle number signal 110 by power cycle curve data which shows a relationbetween the temperature change amplitude and a power cycle life of thesemiconductor device and is stored in a power cycle curve data storagepart 14.

One example of characteristics of a power cycle curve stored in thepower cycle curve data storage part in the electric motor controlapparatus according to the first embodiment is shown in FIG. 5. In FIG.5, the axis of abscissa is temperature change amplitude ΔT (thetemperature change amplitude signal 109 outputted from the thermalstress computation part 13) and the axis of ordinate is the number S ofpower cycles (the power cycle number signal 110 outputted to the thermalstress computation part 13) used as the power cycle life of thesemiconductor device. There is a correlation between the temperaturechange amplitude and the power cycle life of the semiconductor device,and the power cycle life becomes short (S1<S2) as the temperature changeamplitude is large (ΔT1>ΔT2).

Also, the thermal stress computation part 13 receives the number S1 ofpower cycles obtained corresponding to the temperature change amplitudeΔT1 as the power cycle number signal 110, and computes a thermal stresscoefficient x1 by Formula (2), and outputs its result to a lifeestimation part 15 a as a thermal stress signal 111.x1=1/S 1  (2)

Subsequently, in like manner, the numbers S2, S3, S4, . . . of powercycles corresponding to the temperature change amplitudes ΔT2, ΔT3, ΔT4,. . . are received from the power cycle curve data storage part 14 andthermal stress coefficients x2(=1/S2), x3(=1/S3), x4(=1/S4), arecomputed and outputs are produced to the life estimation part 15 a asthermal stress signals 111.

When the thermal stress coefficient is inputted as the thermal stresssignal 111, the life estimation part 15 a obtains a summative thermalstress coefficient X by addition as shown in Formula (3) and outputs thesummative thermal stress coefficient to a display part 16 as a lifeestimation result signal 112.X=X0(previous value)+x1+x2+x3+x4+  (3)

Also, the case of the summative thermal stress coefficient X=1corresponds to a life, so that when the summative thermal stresscoefficient X approximates to 1, an alarm is outputted to the displaypart 16 as the life estimation result signal 112 and a worker isinformed that the life is near.

Also, the life estimation part 15 a receives set time Δt set by anoperating time setting part 17 as a set time signal 113, and receivesand sums the thermal stress coefficients x1, x2, x3, . . . , xn by thethermal stress signals 111 outputted from the thermal stress computationpart 13 for only the set time Δt, and obtains a thermal stresscoefficient Xt per set time Δt by Formula (4).Xt=x1+x2+ . . . +xn  (4)

Also, life time tL is calculated by Formula (5), and an expected lifesignal 114 which is an expected life te set by an expected life settingpart 18 is compared with the life time tL calculated by Formula (5) andin the case of tL<te, an alarm is outputted to the display part 16 as alife determination signal 115 and a worker is prompted to takelife-prolonging measures.tL=Δt×(1/Xt)  (5)

As described above, according to the first embodiment, it is constructedso as to calculate a power cycle life by referring to power cycle curvedata specific to a semiconductor device every temperature changeamplitude computed by the temperature change estimation part, so thatwith respect to any temperature change amplitude, thermal stressweighted based on its amplitude can be computed and summed, and lifeestimation with high accuracy and determination of an expected life canbe implemented.

Second Embodiment

FIG. 6 is a diagram showing a configuration of an electric motor controlapparatus according to a second embodiment of this invention. In thediagram, numerals 1, 2, 4 to 11, 13, 14, 16 to 18, 100 to 115 aresimilar to those of FIG. 1 and the description is omitted.

A life estimation part 15 b outputs a life estimation result signal 112and a life determination signal 115 to a carrier frequency setting part3 b and a current limit level adjusting part 12 b.

When a summative thermal stress coefficient X included in the inputtedlife estimation result signal 112 approximates to 1, or when the lifedetermination signal 115 outputted in the case that life time tL fallsbelow an expected life te is inputted, the current limit level adjustingpart 12 b makes an automatic adjustment so as to decrease a currentlimit value signal 106 outputted to a current computation part 9.

When a life of a semiconductor device such as power transistors 6 anddiodes 7 becomes short, an automatic adjustment is made so as todecrease the current limit value signal 106 and an output current I islimited rather low and thereby the steady-state loss Ps and theswitching loss Psw shown in FIGS. 2 and 3 can be decreased, so that aheating value Q of the semiconductor device can be suppressed. As aresult of this, even when a worker does not inspect the electric motorcontrol apparatus and does not take life-prolonging measures, anautomatic adjustment can be made so as to satisfy an expected life ofthe semiconductor device set by an expected life setting part 18.

Also, when a summative thermal stress coefficient X included in theinputted life estimation result signal 112 approximates to 1, or whenthe life determination signal 115 outputted in the case that life timetL falls below an expected life te is inputted, the carrier frequencysetting part 3 b makes an automatic adjustment so as to lower an upperlimit value of a carrier frequency and outputs a carrier frequencysignal 101 to a control part 1.

When a life of the semiconductor device such as power transistors 6 anddiodes 7 becomes short, an automatic adjustment is made so as to lowerthe upper limit value of the carrier frequency, so that a term of theswitching loss Psw of Formula (1) by which a heating value Q of thesemiconductor device is obtained can be decreased and the heating valueQ of the semiconductor device can be suppressed. As a result of this,even when a worker does not inspect the electric motor control apparatusand does not take life-prolonging measures, an automatic adjustment canbe made so as to satisfy the expected life of the semiconductor deviceset by the expected life setting part 18.

Also, when it is decided that a life of the semiconductor device becomesshort and the carrier frequency is lowered by an automatic adjustment,noise of an electric motor increases, so that a worker can be promptedby being informed that the life of the semiconductor device is near, andthe worker can take measures to replace the electric motor controlapparatus before an abnormal stop of a system is made due to a faultcaused by the life of the semiconductor device.

In the second embodiment, since it is constructed so that the heatingvalue Q of the semiconductor device is suppressed by making an automaticadjustment so as to decrease the current limit value signal 106 and alsomaking an automatic adjustment so as to lower the upper limit value ofthe carrier frequency when a life of the semiconductor device becomesshort, a system can be prevented from stopping before a worker inspectsan alarm displayed by the display part 16 even when the semiconductordevice results in the life rapidly due to thermal stress caused bysudden variations in an output current occurring in the case of acompressor etc. with large variations in a load. Also, even when theworker does not inspect the electric motor control apparatus and doesnot take life-prolonging measures, an automatic adjustment can be madeso as to satisfy an expected life of the semiconductor device set by theexpected life setting part 18.

Industrial Applicability

As described above, an electric motor control apparatus of the presentinvention can implement life estimation with high accuracy anddetermination of an expected life, so that the electric motor controlapparatus is suitable for application in which start and stop control isperformed frequently. Also, even when a worker does not inspect adisplay part and does not take life-prolonging measures, an automaticadjustment can be made so as to satisfy an expected life of asemiconductor device set by an expected life setting part, so that theelectric motor control apparatus is suitable for application in which adecrease in speed of operation is allowed with respect to command speed.

1. An electric motor control apparatus for converting DC electric power into AC electric power of a variable frequency and a variable voltage and performing variable control of an electric motor acting as a load, comprising: a switching circuit having a semiconductor device such as a power transistor and a diode connected in parallel with this power transistor, a control part for generating a driving pulse based on an operating frequency signal set by an operating frequency setting part and a carrier frequency signal set by a carrier frequency setting part, and a driving circuit for amplifying the driving pulse outputted from this control part and performing on-off control of the power transistor of the switching circuit, the electric motor control apparatus characterized by comprising: a current computation part for computing an output current from a current flowing through the semiconductor device and also outputting a current breaking signal to the control part when an output current signal computed exceeds a current limit value signal outputted from a current limit level adjusting part, a temperature change estimation part for estimating changes in temperature of the semiconductor device to compute temperature change amplitude based on this output current signal, the operating frequency signal and the carrier frequency signal, a power cycle curve data storage part for storing power cycle curve data showing a relation between the temperature change amplitude and a power cycle life of the semiconductor device, a thermal stress computation part for converting the temperature change amplitude computed by the temperature change estimation part into the number of power cycles used as the power cycle life of the semiconductor device by the power cycle curve data and computing a thermal stress signal, and a life estimation part for doing life estimation of the semiconductor device based on this thermal stress signal and producing an output to a display part as a life estimation result signal and further calculating life time per set time and comparing the life time with an expected life and outputting an alarm to the display part as a life determination signal when the life time is shorter than the expected life.
 2. An electric motor control apparatus as claimed in claim 1, characterized in that it is constructed so that the life estimation part outputs the life estimation result signal and the life determination signal to the current limit level adjusting part and also the current limit level adjusting part makes an automatic adjustment so as to decrease a current limit value signal outputted to the current computation part when alarm information is included in the life estimation result signal or the life determination signal is inputted.
 3. An electric motor control apparatus as claimed in claim 1, characterized in that it is constructed so that the life estimation part outputs the life estimation result signal and the life determination signal to the carrier frequency setting part and also the carrier frequency setting part makes an automatic adjustment so as to lower an upper limit value of the carrier frequency and outputs a carrier frequency signal to the control part when alarm information is included in the life estimation result signal or the life determination signal is inputted. 